1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting wire, and more particularly, it relates to improvement for increasing the critical current density as well as the length of an oxide superconducting wire.
2. Description of the Background Art
In recent years, superconductive materials of ceramics, i.e., oxide superconductive materials, are watched with interest due to higher critical temperatures thereof. For example, bismuth, yttrium and thallium oxide superconductive materials, which exhibit high critical temperatures of about 110.degree. K., 90.degree. K. and 120.degree. K. respectively, are expected for practical application.
For example, it is known that a bismuth oxide superconductor contains phases having critical temperatures of 110.degree. K., 80.degree. K. and 10.degree. K. respectively. It is also known that the 110.degree. K. phase has a 2223 composition in a composition of Bi-Sr-Ca-Cu or (Bi, Pb)-Sr-Ca-Cu with Bi being partially replaced by Pb, while the 80.degree. K. phase has a 2212 composition in the same composition.
In a method of preparing an oxide superconductor, an oxide superconductor or raw material powder therefor is filled up in a metal sheath and subjected to deformation processing and heat treatment, so that the oxide superconductor or the raw material powder contained in the metal sheath is sintered and brought into a superconducting state. This method is advantageously applied to preparation of a long superconducting wire, for example.
In order to apply an elongated oxide superconducting wire to a cable or a magnet, it is necessary to provide a high critical current density uniformly along its longitudinal direction. When an oxide superconducting wire such as a bismuth oxide superconducting wire is prepared, for example, raw material powder is filled up in a metal sheath, worked into a tape by first rolling and then heat treated, to obtain plate-type oxide superconductor particles of several 10 lm in length. Then, second rolling is so performed as to orient the plate-type particles in the same direction and bring grain boundaries therebetween into close contact with each other. When such second rolling is performed, therefore, loads are vertically and horizontally applied to the tape surface. Thus, cracking may be caused in the interior of the superconductor portion to reduce the current density.